If you want build a simple Bluetooth amplifier board. AudioB plus Bluetooth audio receiver is a good choice for your project. This is a bluetooth 4.0 stereo recevier. Based on CSR8635. We breakout pins to 2x10 2mm space male pins. You can integrate it to your project easily. This module has a U.FL connector. You can connect an external 2.4G antenna with it by using a U.FL to SMA cable.

Single-ended signal output with Audio amplifer

Single-ended signal output with headphone/small speakers

Differential signal output

Microphone input

Note:

Quick press S2 to switch to the previous song, long press to reduce the volume

Quick press S3 to switch to the next song, long press to increase the volume

Press S1 to play or pause

if there have audio output, MUTE pin will be high level, otherwise, will be low level.